The invention relates to a color display device comprising a color display tube on a longitudinal axis of which there is arranged:
a) a neck, a cone, a display screen provided with an arrangement of phosphor dots luminescing in different colors, and a color selection electrode which is situated opposite said display screen.
b) the neck accommodates an electron gun having a beam-forming part for generating three electron beams which are to be driven by video information and the axes of which extend in one plane, and
c) a deflection unit for generating deflection fields which serve to deflect the electron beams in the horizontal and vertical directions, which deflection unit defines a deflection plane.
Color display tube systems of the type mentioned in the opening paragraph are of the conventional 3-in-line type. In general, they comprise a self-convergent deflection unit which, in operation, generates such non-uniform magnetic fields for horizontal and vertical deflection (in particular a barrel-shaped field for vertical deflection and a cushion-shaped field for horizontal deflection) that the three electron beams generated by the electron gun and focused on the display screen by the main lens converge throughout the display window.
Nowadays ever higher beam currents are required, which can be partly attributed to the use of darker glass. A problem associated with said higher beam currents is that the beams repel each other, so that it is impossible to provide a perfect picture for all beam-current values. In addition, the picture performance must meet ever stricter requirements.
The problem is being dealt with, for example, by correcting convergence errors caused by space-charge repellence or by precluding space-charge repellence by making sure that the three beams do not coincide with respect to time.
The invention is based on the recognition that space-charge repellence between the beams does not only cause a convergence error but also a landing error. If the convergence error caused by space-charge repellence is corrected, the landing is adversely affected and cannot be corrected. To optimize the correction, first the landing influences of space-charge repellence must be corrected, whereafter it is possible to correct, if necessary, the convergence which may have been adversely affected by said correction.
The expression "correct landing" is to be understood to mean in this context a correct landing angle. The three electron beams which must drive the phosphor dots of a specific pixel must pass through the same mask hole) of the color selection electrode (shadow mask) at slightly different, predetermined angles. Space-charge repellence experienced by the (outermost) beams has a disturbing effect on these angles. It is an object of the invention to correct the space-charge influence which affects the landing angles.
This object is achieved by a color display tube system in accordance with the invention, which is characterized in that an element influencing the landing angle so as to correct the effect of space-charge repellence experienced by the (outermost) beams is provided in an axis-position between the axis-positions of the beam-forming part of the electron gun and the display screen. The correcting operation can be carried out in various ways. It is possible, for example, to measure the cathode currents or beam currents, and to derive from the measured information information how to energize the element for influencing the landing angle. A very practical solution is deriving the information from the video information.
A preferred embodiment in accordance with the invention is characterized, in that a correction device is provided which supplies a correction signal to the element influencing the landing angle, said correction signal being derived from the video information.
In this preferred embodiment, a number of drive modes are possible. The correction signal can be derived from the instantaneous video information (this requires a rapid correction circuit). The correction signal can be derived from the average beam current per line (to start from the correct line, a line memory is necessary). The correction signal can be derived from the average beam current per picture (which is known from the preceding picture or pictures).
All this is based on the following. If the value of all beam currents to be used is known, then the mutual repellence of the beams can be calculated for all these beam currents. If the mutual repellence is known, then the degree to which the landing angle is influenced is known too. This can be used to determine the necessary correction signal. By supplying said correction signal to the element influencing the landing angle, the outermost electron beams experience, during operation, a force causing these electron beams to be displaced relative to the central beam. This correction signal is adjusted such that the effect on the landing angle of the electron beams substantially compensates for the effect of beam repellence. The object of the invention is achieved in that the arrangement of the element influencing landing in an axis-position between the display screen and the deflection plane, causes the element to exert a force, in the case of a correction, on the outermost electron beam(s), which force comprises a component extending in the plane of the electron beams and in a direction towards the central electron beam, and the arrangement of the element influencing landing in an axis-position between the deflection plane and the beam-forming part of the gun, causes the element to exert a force, in the case of correction, on the outermost electron beam(s), which force comprises a component extending in the plane of the electron beams and in a direction away from the central electron beam. In the latter case, the sensitivity of the correction system is the highest (which means that the drive currents necessary are the lowest).
The magnetic fields to be generated for the desired influencing processes may be local dipole fields at the location of each one of the two outer beams.
To ensure that the electron beams can be focused to a sufficient degree, a preferred embodiment of the invention is characterized in that the element influencing the landing angle is constructed in such a way as to generate a 45.degree. magnetic 4-pole field. (Particularly if said element is arranged near the focusing lens of the electron gun).
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.